The Ancient Weapons (古代兵器 Kodai Heiki?) are three weapons capable of mass destruction, each taking a different form. They are known as Pluton, Poseidon, and Uranus, each named after a deity for their godly powers. The weapons are not limited only to inanimate objects, but consist of living beings as well.
The Ancient Weapons are the argument used by the World Government to forbid research of the Void Century, since they fear that knowledge of the weapons could result in a global war. However, this has not stopped members of the World Government, most notably Spandam and former Warlord Crocodile, from attempting to gain one of the weapons for themselves. Even without knowledge of the missing years, their power has been known to entice casual observers (as was the case with Vander Decken IX's interest in Shirahoshi, otherwise known as the Ancient Weapon Poseidon).
- Further information: Pluton
While Pluton has yet to appear in the story, it has been referred to as an ancient ship capable of massive damage and destruction. It was built on the island of Water 7 sometime during the Void Century, and the shipwrights kept the blueprints as a safety measure. In the event that Pluton fell into the wrong hands, another one can be built to fight the existing one. Fortunately, the ship was lost to history before that could happen. The blueprints were passed down through the generations, and ten years prior to the main storyline were held by the fish-man carpenter, Tom, of Tom's Workers.
At this point, Spandam sought him out in order to acquire the blueprints so that they could use Pluton to put an end to the Great Pirate Era (in reality, it was to overthrow the World Government). Tom denied the request, and shortly afterwards handed them down to his apprentice Iceburg, who passed them on to Tom's other apprentice Cutty Flam, otherwise known as Franky, five years later in order to better hide them. When Franky was captured by the World Government, he kept the blueprints hidden within his own body until he understood that Nico Robin, the only living person who could read the Poneglyphs and subsequently revive the Ancient Weapons, had no intention of doing so or even planned on using that power to destroy the world; he subsequently burned the blueprints. While the exact whereabouts of Pluton remain unknown, the Arabasta Poneglyph contains information on its location.
- Further information: Poseidon
Poseidon was first mentioned when Nico Robin read the Poneglyph on Skypiea. However, details of the weapon were not delved into until much later, when she reached Fish-Man Island. Poseidon was a Mermaid Princess who lived in the Void Century that had the power to talk to Sea Kings, enormous sea creatures capable of mass destruction. Princess Shirahoshi also has this power, and as stated in a conversation between Neptune and Robin, she has now inherited the title "Poseidon".
- Further information: Uranus
- Each of the Ancient Weapons is named after a Greco-Roman god affiliated with the earth, sea, and sky respectively, and they each have some reference to said weapon:
- Pluto ruled the underworld, which was believed to be underground and all mineral wealth within the earth belonged to him; Pluton is said to be buried deep somewhere in Arabasta.
- Poseidon was the ruler of the sea; Shirahoshi is the current Poseidon and a mermaid princess.
- Uranus was the personification of the sky; currently has no reference yet since it's identity is unknown.
- The Marines' trump card, the Dyna Stone, is said to be a weapon with power on par with the Ancient Weapons.
- Nico Robin is the only one to know the location and capabilities of two of the Ancient Weapons: Pluton and Poseidon. It is unknown if she knows anything about Uranus.
- One Piece Manga and Anime — Vol. 62 Chapter 612 (p. 17-19) and Episode 531, Shirahoshi is finally seen for the first time.
- One Piece Manga and Anime — Vol. 66 Chapter 650 and Episode 570, Robin and Neptune discuss about the three Ancient Weapons named after Gods.
- One Piece Manga and Anime — Vol. 66 Chapter 649 and Episode 569, Robin and Neptune speak of Shirahoshi and Poseidon.
- One Piece Movie 12, The Dyna Stone's power is comparable to the Ancient Weapons.